Gas sensing system

ABSTRACT

A gas sensing system is provided for the detection of gas concentrations in a flowable medium. The gas sensing system has a gas permeable membrane structure to separate the flowable medium from a chamber where a gas sensor is positioned for detecting the gas concentration in the chamber. The sensor has predetermined environmental operating ranges and the system has a heat drain with a heat sink to maintain the temperature of gas in the chamber within the operating ranges, together with gas nozzles leading into the chamber to purge gas in the chamber and to maintain the gas concentration from exceeding the sensor operating range.

The present invention relates to a gas sensing system and was primarilydeveloped for measuring gas concentrations, in solution, in gasmixtures, liquids and other fluid mediums, for example foodstuffs suchas jams and other preserves, and for carrying out such measurementsin-situ, preferably during the continuous flow of the medium. The systemalso has applications for gas sensing in other media, such as wine,beer, cider, sugar solutions, flowable solid material such as sugar,food granules and agglomerates together with non-foodstuffs such aspharmaceutical preparations and oils.

It is often important to know the concentrations of certain gases incertain mediums, and one of the most important applications is indetermining the acceptability of foodstuffs. A prime example arises inthe preserves industry where sulphur dioxide gas, SO₂, is frequentlyemployed during storage and transportation as a preservative for the rawmaterials (for example, pulped fruit) from which the preserve is made.Once the preserve has been manufactured the sulphur dioxide may nolonger be required in the finished product and, in fact, EuropeanCommunity Regulations are in force requiring that sulphur dioxideconcentrations in preserves be less than thirty parts per million (ppm).In other foodstuffs and beverages (such as beer), legislation world-wideis becoming stricter and present U.S. requirements limit the SO₂concentrations in beer to less than 10 ppm. However, in certain productssuch as wine, the sulphur dioxide is required, in known concentrations,to act as a preservative during storage. In both cases, it is highlydesirable to know the concentration level of the sulphur dioxide.

The conventional procedure for regulating gas concentrations inpreserves is by "boiling off" the gas by heating, or by vacuumprocessing the preserve sufficiently to cause the gas to evolve fromsolution until its concentration falls to a desired level. Theconditions under which the boiling off procedure is effective will varydepending on the concentrations of the gas in the preserve and how it ischemically bound with the preserve (it is known that sulphur dioxideforms weak complexes with certain naturally occurring organic materials,particularly aldehydes and ketones, and such complexes are generallybroken down at temperatures in excess of 60° C.) and therefore it isinevitable that the residual gas concentration in the finished productwill vary.

Therefore, quality control checks are necessary at frequent intervalswhereby samples of the preserve are taken and tested, using standardlaboratory procedures, to determine the gas concentrations in thepreserve product. In the production environment, known gas sensors areneither sufficiently rugged or suitable for use at the high temperaturesand pressures generally required. Quality control regulations mayrequire that certain gas concentrations, such as sulphur dioxide, in afoodstuff product may not exceed a permitted level. Furthermore, iffollowing a quality control check on sulphur dioxide concentration it isfound that the gas concentration level is too great, a continuousproduction line of the foodstuff may be stopped to determine the originof the problem while the gas removal procedure is revised. Typically,the known method of gas concentration measurements (such as SO₂) isoff-line, whereby a production sample is taken and gas concentrationmeasurements are achieved by wet chemical means, a destructive test,taking between 30-60 minutes. During this period, production continuesat the rate of several tonnes per hour. Therefore, if an unacceptablegas concentration is found then several tonnes of the product will havebeen produced which will be suspect and require further processing.Furthermore, simply increasing the boiling off time of the preserve toensure that more gas will be removed, without control, would likelyresult in an unacceptable increase in production costs.

In addition, existing tests, even using the most accurate commerciallyavailable wet chemical testing methods (which take 40 minutes), resultin test results showing a standard deviation of about 4 ppm whenrepeated measurements are taken of the same commercial sample having aconcentration in the region off 10 ppm. Such a wide fluctuation of datafrom the test results where extremely small quantities are involved isdetrimental to overall manufacturing capabilities, the risk existingthat incorrect measurements (due simply to lack of measurement accuracy)could result is unacceptable product being declared acceptable andacceptable product being re-processed.

It is an object of the present invention to provide a gas sensing systemwhich alleviates the aforementioned problems and enables gasconcentration measurements to be carried out in-situ.

According to the present invention there is provided a gas sensingsystem for the detection of gas concentrations emanating from a flowablemedium which comprises a membrane structure permeable to said gas anddisposed between said flowable medium and a chamber, said membranestructure being impervious to said flowable material; a gas sensorexposed to said chamber for the detection of concentrations of said gastherein, said gas sensor having predetermined environmental operatingranges and wherein means is provided in said chamber whereby thephysical characteristics of the gas in the chamber are controlled forthe characteristics of the gas exposed to said sensor to be compatiblewith the predetermined operating ranges of the sensor. Such a systemallows gas concentration measurements to be carried out on the medium insitu, enabling continuous testing of the medium which allows for greaterprocess and quality control and provides the opportunity to blendseparate batches of the medium.

Conventional gas sensors may have an operational temperature limitationless than a predetermined temperature and with such sensors atemperature control means may be provided, preferably within thechamber, for withdrawing heat from the chamber and the gas that entersthe chamber through the membrane structure, thereby reducing thetemperature of the gas within the chamber that is presented to thesensor to below the predetermined temperature. Preferably, thetemperature control means comprises a heat sink such as a thermallyconductive member through which heat from the chamber is conducted to aheat sink outside of the chamber. Usually, conduction means will also beemployed to control the temperature of the sensor below thepredetermined temperature, by conducting heat away from the gas sensingsystem, preferably such conducting means incorporating a heat sink. Theuse of such temperature control means also alleviates heat transferthrough the gas sensing system to the sensor by conduction, therebyhelping reduce the heating of the gas sensor beyond its operating rangeas a result of such heat conduction through the system.

Alternatively, or in addition, conventional gas sensors may have anoperational gas concentration detection limitation which is less than apredetermined concentration for such sensors to provide increaseddetection accuracy or a high sensitivity. With such sensors purgingmeans may be provided to reduce the concentration of gas within thechamber, as necessary, to maintain that gas concentration in the chamberbelow the predetermined concentration. Preferably, the chamber is sealedand the purging means may comprise inlet and outlet ports, an input portpermitting the controlled introduction of a purging gas into the chamberand an outlet port permitting the mixed gaseous contents of the chamberto be removed from the chamber so that said chamber is filled with thepurging gas.

Preferably, the system will comprise a pH sensor for detecting the pHlevel of the flowable medium. Typically, the pH sensor will be a pH IonSensitive Field Effect Transistor and such sensors advantageously serveto provide an accurate pH reading of the medium, which readings may beimportant in determining the actual gas concentrations in solutionwithin the medium from the gas sensor reading, since the gasconcentrations emanating from the medium may be proportional to the pHlevel of the medium.

In a preferred form of the present invention the system furthercomprises pressure detection means for determining the pressure of theflowable medium and a temperature detection means for determining thetemperature of the flowable medium, either or both of which mayadvantageously serve in determining the physical state of the medium,basically its temperature and pressure, which may be important indetermining the actual gas concentration in solution within the mediumfrom the gas sensor reading, since the gas concentrations emanating fromthe medium may be proportional to the pressure and/or the temperature ofthe medium. In the case of SO₂ in preserves, for temperatures below 85°C. there is a significant temperature dependence of the gasconcentration emanating from the medium. Above this temperature of 85°C. the complexes formed between the SO₂ and the organic materials arebroken. This temperature dependency is also dependant on the organicmaterial (and thus the complexes formed therewith) and is, for example,different for SO₂ in wine or SO₂ in beer. Above this complexationtemperature test results have shown that the concentration of SO₂emanating from preserves also varies with pressure variations and withvariations of the preserve pH level as well as with further temperaturevariations.

The release of gas in solution in a medium is proportional to thephysical characteristics of the medium, particularly temperature andpressure, and variations of these parameters will cause a variation ofthe concentration of gas released. By measuring the gas concentrationsemanating from a medium and knowing the physical characteristics of themedium it is possible to determine the actual concentration of the gasin solution in the medium.

Typically, the flowable medium will flow through a pipe, in a wall ofwhich is situated the membrane structure, so that a side face of themembrane structure is directed inwards towards a central axis of thepipe and usually it will be in contact with the medium. The membranestructure may be supported by a lattice frame to increase its mechanicalstrength. This membrane structure may comprise a single layer or amultiplicity of two or more layers.

The gas sensor may be of conventional structure and have a second gaspermeable membrane structure disposed between the gas detectioncomponents of the sensor and the chamber.

The gas sensing system is preferably intended for detection of gasconcentrations in solution in a non-gaseous flowable medium which willtypically be liquid, paste, gel, powder or granules.

Further according to the present invention there is provided a method ofdetermining concentration of a gas emanating from a flowable medium byuse of a gas sensor having predetermined environmental operating ranges,comprising the steps of separating and isolating the said gas from theflowable medium and controlling the physical characteristics of the saidisolated gas for said gas to be presented to the gas sensor in acondition in which its physical characteristics are compatible with thepredetermined operating ranges of the gas sensor.

Usually, the gas will be separated from the flowable medium by locationof a membrane structure, which is permeable to the gas and impermeableto the flowable medium, between said flowable medium and a chamber andallowing the gas to diffuse into the chamber for detection therein bythe gas sensor.

The sensor used may have an operational temperature limitation less thana predetermined temperature and in such case when the gas emanating froma hot medium is at a temperature greater than the predeterminedtemperature, by the method of this invention heat may be withdrawn fromthe gas in the chamber to reduce the temperature of that gas to belowthe predetermined temperature for its exposure to the gas sensor,thereby adjusting the temperature of the gas to within the operatingranges of the sensor. As a further example, the sensor may have anoperational concentration detection limitation less than a predeterminedconcentration and in such a case when the gas emanating from a medium isat a concentration greater than the predetermined concentration, by themethod of this invention the concentration of the gas in the chamber maybe controlled and prevented from reaching the predeterminedconcentration for its exposure to the gas sensor.

Primarily developed for sulphur dioxide concentration detection inpreserves such as jam etc., it will be appreciated that this inventioncan also be applied to a large number of other situations, for examplesulphur dioxide in wine production, where it is necessary to determine agas concentration in solution (not necessarily sulphur dioxide) of aflowable medium.

In addition, it will also be appreciated that, optionally, thedetermining of the gas concentrations in solution of a flowable mediumin accordance with the present invention can be carried out with themedium in a static state or when it is flowing.

An embodiment of a gas sensing system constructed in accordance with thepresent invention will now be described, by way of example only, withreference to the accompanying illustrative drawings in which:

FIG. 1 is a cross sectional view of a gas sensing unit attached to apipe of the system;

FIG. 2 is an enlarged view of the membrane support means indicatedgenerally at A in FIG. 1;

FIG. 3 is a plan view from above of the gas sensing unit of FIG. 1;

FIG. 4 is a cross sectional view of a second embodiment of a gas sensingunit;

FIG. 5 is a schematic view of a gas sensing system of FIG. 1 connectedto a continuous flow production pipe for a medium under consideration;

FIG. 6 is a graph showing the change of gas concentration, within thegas sensing unit of the system, with time; and

FIG. 7 is a graph of gas concentration, within the gas sensing unit ofthe system, with time and in which system a gas purging operation isemployed.

FIG. 8 is a schematic view of a purging system used in connection withthe invention.

A gas sensing system 10 (FIG. 5) comprises a gas sensing unit 12 (FIG.1), connected to a mounting block 13 by bolts 16 and 17 (FIG. 3), withthis mounting block 13 surrounding and clamped onto an aluminium pipe14, a pressure transducer 18 for determining pressure within the pipe 14and a pH sensor head 20 for determining the pH level of the contents ofthe pipe 14. A longitudinal array of conventional thermocouples (notshown) are disposed within the pipe 14 for determining the temperatureof the pipe contents.

In the system shown in FIG. 5 the pipe 14 is a bypass to a mainproduction flow pipeline 30 and 3-way isolation valves 32 are employedto optionally a) close flow to the bypass pipe 14, b) permit flowthrough both the main pipe 30 and the bypass pipe 14 or c) to direct theflow through the bypass pipe 14 only (depending on the requiredoperational procedure). However, it will be appreciated that the gassensing system could alternatively be fitted directly to the main pipe32 if required. The direction of flow of the pipe contents are shown bythe arrows 34. A conventional temperature sensor 31, such as one or morethermocouples, optionally may be included in pipe 32, as shown, tomeasure the temperature of the pipe contents.

A drain valve 40 and a flow restriction valve 42 are positioned on thebypass pipe 14 to permit, if required, the flow rate of the contents ofthe bypass pipeline 14 to be varied by operation of the appropriatevalve. By closing valve 42 the flow rate may be decreased while openingvalve 40 will enable the contents to be discharged. These valves 40 and42 are not essential to the working of the gas sensing system.

The pressure transducer 18 is of conventional form while the pH sensor20 may employ a standard pH Ion Sensitive Field Effect Transistor whichis a rugged sensor designed for operation in extreme operatingconditions. Both are well known and commonly available and, as such,will not be discussed further herein.

The system of the present example will be considered for detecting theconcentration of sulphur dioxide (SO₂) in a preserve such as jam,sulphur dioxide being commonly employed as a preservative in thoseproducts from which the preserve is formed. SO₂ can also reside in foodor other products as a result of natural production, steriliser residuesfrom apparatus cleansing processes, residue from food bleachingprocesses or even from processes to modify the physical or mechanicalcharacteristics of foodstuffs. However, it will be appreciated that itis desirable to know, whatever the source of the gas content in theproduct, the concentrations of such gases. With this in mind, the gassensing unit 12 comprises a conventional and known sulphur dioxide(electrochemical) gas sensor 50 mounted in a stainless steel sensorhousing 52. The sensor 50 (which may be that sold under the Trade MarkSIEGER) will usually comprise a potassium hydroxide gel which reactswith sulphur dioxide to provide an electrical output which is indicativeof the sulphur dioxide concentration present. This sensor is capable ofselectively detecting sulphur dioxide in the presence of other gasses.The gas sensitive sensor components are usually protected by a gaspermeable sensor membrane (not shown). Such sensor technology is readilyavailable and will not be discussed further herein.

The region of the pipe 14 on which the sensing unit 12 is mounted has anaperture 68 extending therethrough which is aligned with an aperture 69in the mounting block 13, providing communication between the interiorof the pipe 14 and the sensing unit 12. Mounted across the aperture 69of the mounting block 13, in sealing engagement, is a gas permeablemembrane 60 (permeable to sulphur dioxide gas) (FIG. 2 showing anenlarged view of part A of FIG. 1 show this membrane 60). This membrane60 is supported on both faces thereof by sandwiching it between inner 61and outer 62 stainless steel mesh retainer plates 64 which permitsulphur dioxide to pass therethrough and through the membrane 60. Themembrane 60 is impermeable to the liquid contents of the pipe 14 whileallowing sulphur dioxide gas therein to pass through. Such gas permeablemembranes are well known and widely available, and the structure of suchmembranes may comprise a single layer or a multiplicity of layers. Knowngas permeable membranes will also permit other gasses to pass through aswell as sulphur dioxide but the sensor 50 employed within the system iscapable of selectively detecting the sulphur dioxide gas within thechamber 84.

A nickel plated copper heat drain 86 has a recess 85 in which the sensor50 is accommodated, and an upper aperture of this recess 85 is sealedwith the sensor housing 52 to form an upper chamber 94, in which thesensor 50 is housed. This upper chamber 94 is effectively sealed byairtight silicon rubber gasket seals 77 and 79 which are respectivelyengaged between the sensor 50 and the heat drain 86 and the heat drain86 and the sensor housing 52.

The seal 77 has an aperture 81 extending therethrough providingcommunication between the sensor 50 and a main, lower chamber 84. Thislower chamber 84 is defined between the seal 77 (and sensor 50) and thegas permeable membrane 60. The heat drain 86 extends to define sidewalls of this second chamber 84 and has perforated arms 87 extendingthrough the chamber 84 to increase the surface area of the heat drainexposed to a gas within the chamber to increase the heat dissipationaway from such a gas. These arms to not restrict gas diffusion withinthe chamber 84. The heat drain 86 is retained clear of the membrane 60by a silicon rubber (insulated) gasket seal 88, And this insulator 88also serve to support the heat drain 86 clear of the mounting block 13to prevent direct heating of the heat drain 86 by conduction from themounting block 13 (such direct heat conduction from the pipe, though themounting block to the heat drain could reduce the capability of the heatdrain to conduct heat away from the gas within the chamber 84). Thisgasket seal 88 further serves to restrict heat conduction from themounting block 13 to the unit 12. The heat drain 86 extends from thechamber 84 and is held in contact with a heat sink 54 which is mountedexternally of the chamber 84 on the sensor housing 52 by the bolts 16and 17. The heat sink 54 includes an array of heat dissipating fins orflanges 56 extending away from the housing 52 and which are intended tobe simply cooled by the flow of air thereover. In addition, the heatsink also serves to cool the body of the gas sensing unit 12 byconducting heat away from the unit 12.

The insulated, annular and airtight seal 77 is also used to support thesensor 50 (which is connected to the sensor housing 52) in the chamber94 clear of the heat sink 54 thus restricting heat conduction from theheat sink to the sensor 50, as well as defining the chamber 84 betweenthis seal 77 and the membrane 60.

In practice, the bolts 17 pass through the sensor housing 52 and theheat sink 54 to engage the heat drain 84 and are tightened to compressthese parts of the unit together, compressing the housing 52 against theupper seal 79. The bolts 16 pass through the heat sink 54 and the heatdrain 86 to engage with the mounting block 13 to compress the unit 12towards the mounting block 13 so that the heat drain 86 is held securelyagainst the seal 88. The components carried by the bolts 16 and 17 canbe slidably received thereon to allow the unit to be easily dismantledand the bolts 16 and 17 may then tightened to compress the componentstogether against the respective seals 79 and 88 to form an airtightchambers 84 and 94.

As shown in FIG. 8, provided with the chamber 84 are two gas ports 300,302 each of which is connected by ducts to controlled valves 310, 312.One of these gas ports may be to a known pressurised gas supply 304, 306(such as an inert gas, a control gas or air) whilst the other may beconnected to a lower pressure system which may be atmosphere or a vacuum308. By this arrangement, gaseous contents of the chamber 84 can bepurged by opening the gas ports to positively charge the chamber 84 withan inert gas, or by a control gas of known concentration 309 for thepurpose of calibrating the sensor 50. When the sensing system is to beused the chamber 84 is purged of sulphur dioxide and the two gas portsclosed.

A primary use of the gas sensing system 10 is in the preservemanufacturing industry to detect the concentration of sulphur dioxidegas, in solution, in an unset (or molten) preserve prior to the bottlingof the preserve. In such use, the pipe 14 is filled with the fluid(unset) preserve, which preserve will usually be at a high temperature,in excess of 85° C., for boiling off sulphur dioxide which is insolution in the preserve and which gas was initially used as apreservative for the ingredients in the preserve.

Sulphur dioxide forms weak complexes with certain naturally occurringorganic materials in the preserve, particularly aldehydes and ketones.These complexes are stable at lower temperatures and are only brokendown at temperatures greater than 60° C. Therefore, it is necessary toheat the preserve to a temperature of about 85° C. to break down thesecomplexes in order to release the sulphur dioxide from the preserve.

As the preserve flows through the pipe 14 and the unit 10 at atemperature in excess of 85° C., sulphur dioxide gas in solution thereinevolves and passes through the pipe aperture 68 and gas permeablemembrane 60. The membrane 60 is impermeable to the preserve and thesteel plates 61, 62 support the membrane 60 from mechanical stressesapplied thereto by the preserve. The membrane 60 and plates 61, 62 willusually be made of a non-stick material or have a non-stick coating toalleviate fouling of the preserve on these parts. If the molten preserveis stationary within the pipe 14 the sulphur dioxide will evolve fromsolution in the preserve and pass through the membrane 60 untilequilibrium is reached whereby the concentration of the sulphur dioxidewithin the cavity 84 is equal to the concentration of sulphur dioxideevolved from the preserve. This is shown in FIG. 6, showing the rise insulphur dioxide concentration within the chamber 84 with time.

As the preserves are at a high temperature (of the order of 85° C. orgreater) the sulphur dioxide gas evolved is also at this hightemperature. However, known gas sensors 50 are recognised as having anoperational temperature range up to 30°-45° C. and thus could notoperate accurately and possibly be destroyed if subjected to the hotconcentrations of sulphur dioxide gas and thereby raised above theirtemperature limit. Therefore, the hot sulphur dioxide gas which entersthe chamber 84 has its physical characteristic of temperature controlledby passage through the heat drain 86, 87 which conducts excess heat fromthe gas in the chamber 84 to the heat sink 54 where it dissipates. Coolair is sometimes blown across the fins 56 of the heat sink to increasethe heat dissipation. In this manner, the gas in the chamber 84, and thechamber itself, is cooled to a temperature below the maximum operationaltemperature of the gas sensor 50 before it passes through a sensormembrane of the sensor 50 for measurement. The aperture 81 of the seal77 provides gaseous communication between the sensor components and thechamber 84. Additionally, the heat sink serves to conduct heat away fromthe body of the sensor 12 and the walls of the chamber 84 helping tocool the gas therein and alleviate conduction of heat to the sensor 50which could also serve to raise the sensor temperature above itstemperature operating range, and so helping maintain the sensor 50temperature below its operational temperature limit.

The sensor 50 then provides an electrical output which is indicative ofthe concentration of the sulphur dioxide gas within the chamber 84 and,hence, in the preserve. In addition, the pH level of the preserve in theregion of the sensor may be measured together with temperature andpressure of the preserve and, from a combination of this information andcalibration measurements of the sensor, the true concentration of thesulphur dioxide gas in solution in the preserve may be determined.

The gas sensor 50 output is indicative of the gas concentration of thesample for given parameters of temperature, pH, pressure etc. Thisoutput may be calibrated by using complex, slow (but well known) testingtechniques to accurately determine the gas content in such samples andcomparing the accurate results with the sensor output and determininghow such measurements and sensor outputs vary for different operatingconditions (and product). Such known techniques for determining theabsolute concentrations of the gas content in a product sample willinclude mass spectrometry using gas chromatography, liquidchromatography, pyrolysis analysis and cryogenic refocussing techniquesor liquid and ion chromatography techniques. Once the system 10 has beencalibrated by this method then the gas concentrations within the testedproduct may be determined from the sensor output and operatingconditions.

By use of the aforementioned procedure, on line gas concentrationmeasurements can be taken with the results being made available withlittle delay. An obvious benefit of this is that the acceptability ofthe preserve can be determined rapidly and, should the sulphur dioxidegas concentration exceed an acceptable level, appropriate steps can betaken quickly to remedy the situation without incurring a large loss inpreserve production.

The foregoing description of the system 10 considers its use with asample of the preserve being directed into the bypass pipe 14 andretained for stationary testing. However, an alternative use of thesystem 10 may be employed for the continuous measurement of the sulphurdioxide gas concentration with the preserve in a continually flowingstate. In such a case, the gas sensing system 10 is connected directlyto the main production pipe 30. Preferably, however, the sensing system10 is connected in parallel to the bypass pipe 14 as previouslydescribed, but in this situation the preserve is allowed to flow overthe membrane 60 during the measurement procedure. This arrangementallows for maintenance of the sensor unit 12 without the need to haltproduction by simply sealing off the by-pass pipe from the mainproduction pipe 30.

In either of the applications of the system 10 to a continuous flowmeasurement arrangement as described, the preserve is again at atemperature in excess of 85° C. so that the sulphur dioxide that evolvesfrom the preserve will diffuse through the membrane 60 in an effort forthe gas to reach equilibrium across that membrane 60. In one suchscenario, once equilibrium is reached the concentration of sulphurdioxide in the chamber 84 will only vary when the concentration of gaswithin the preserve varies and causes diffusion of the sulphur dioxideacross the membrane 60 in either direction in an effort to maintainequilibrium. Measurements from the sensor 50 can thus be monitored forfluctuations to indicate any possible variations in the gasconcentration of the product.

However, it is preferred that a more sophisticated use of the gassensing system 10 is employed to detect the concentration of sulphurdioxide in a continuous preserve flow. It is possible that the flow rateof the preserve may be such that where a portion of the preserve has aconcentration of sulphur dioxide higher than a normal, or acceptablelevel, it will take some time for equilibrium indicative of the higherlevel concentration to be reached (due to the rate of diffusion slowingdown as equilibrium is approached) and thus the change in concentrationmay not be detected as rapidly as is desirable. To alleviate thisinconvenience, a succession of independent gas concentrationmeasurements can be taken intermittently and continuously usingapparatus similar to that previously described. FIG. 6 shows a graph ofthe increase in concentration of sulphur dioxide within the cavity 84for testing a stationary sample of the preserve (as previouslydescribed) where the increase in concentration within the chamber 84will follow a recognised pattern, in which the concentration initiallyincreases at a high rate and subsequently increases at a progressivelyslower rate as the concentration approaches equilibrium across themembrane 60 after several minutes. However, by the use of standardmathematical extrapolation principles, it is possible to determine thefinal equilibrium gas concentration from a study of the initial slope ofthe graph as shown at region 100 in FIG. 6 after about 10-15 seconds.Thus, the sensing system 10 can be employed to take successive gasconcentration measurements, each over an initial period of 10 to 15seconds from the initiation of the gas diffusing into the chamber 84 inan effort to reach equilibrium (across the membrane 60) after purgingthe chamber 84. From these results, successive equilibriumconcentrations of the preserve can be mathematically determined. Thisobviously offers the benefit of determining the gas concentration in amuch quicker fashion, normally in under twenty seconds compared toseveral minutes normally required for equilibrium to be reached.

To take advantage of the aforementioned principle using successivemeasurements, it is possible that the basic gas sensing system 10 willneed to be modified slightly. The normal gas concentrations of sulphurdioxide within the molten preserve is expected to be less than 30 partsper million (ppm) (although this may be exceeded when the product isdefective) and as such the gas sensor 50 would normally be required tooperate efficiently up to a sulphur dioxide concentration greater than30 ppm. However, for the modified use of the system 10, a more sensitivesensor 50 is preferred for rapidly and accurately determining theinitial increase in gas concentration before equilibrium is reached andfor this a sensor 50 may be employed having a range up to, say, 10 ppmwhich permits rapid and accurate readings to be made up to that lowconcentration. With such a modification it is therefore necessary toprevent the concentration within the chamber 84 from exceeding thesensor limitation of, say, 10 ppm (otherwise it may be that the sensorwill "burn out"). With this in mind, the chamber 84 is purged of thesulphur dioxide periodically (say every 15 seconds) before itsconcentration in the chamber 84 develops to an extent greater than thatrecommended or permitted for the sensor 50. This purging and effectivecontrol in the physical characteristics of the sulphur dioxide in thechamber is achieved by blowing air or an inert gas through one of thegas ports (not shown) which are provided in the chamber 84, and purgingthe mixed gases by way of the second gas port prior to the sulphurdioxide concentration in the chamber 84 exceeding that recommended orpermitted for the sensor 50.

Following each purging stage the two gas ports are closed whilst sulphurdioxide gas in the preserve continues to diffuse through the membrane 60and the initial sulphur dioxide gas concentration level increase inchamber 84 is measured. At the same time pH measurements and temperaturemeasurements of the preserve in the pipe will likely be taken for use incalculating the sulphur dioxide concentration in the preserve. Thepressure of the preserve within the pipe may also be measured ifrequired. The flow rate of the preserve through the pipe will likely besuch that the gas concentration in the preserve will not vary greatlyduring each (approximately) 15 second measurement time so that anaccurate measurement of the gas concentration within the preserve may beexpected. Furthermore, by repeating the purging/measurement processintermittently (see FIG. 7 for a graphical indication of the sulphurdioxide concentration in the cavity 84 during this process ofmeasuring/purging) any variations in the sulphur dioxide concentrationcan be detected extremely quickly and any necessary alterations to themanufacturing process can be effected immediately.

In either of the two uses described, the accuracy of the system isincreased at higher temperatures of the preserve since a higherproportion of the sulphur dioxide is evolved from the preserve, thusenhancing the system sensitivity. However, although heating preserveshas no adverse effect on the final product, in certain other products,for example wine, the quality of the final product will be adverselyaffected by heat. Therefore, a feature of this invention is that gasmeasurements emanating from the product can be measured at a wide rangeof temperatures. For example, in wine production a series of sulphurdioxide measurements may be taken with the wine at a temperature above60° C. using the present system to obtain a series of calibrationresults and similar measurements taken at temperatures well below 60° C.From these two sets of results the relationship between the true sulphurdioxide content and the sulphur dioxide content at an ambienttemperature can be determined, allowing a non-destructive test on themajority of the product. The relationship between true gas content andthe gas content measured at lower temperatures will vary for differentmaterials, but the present system allows for high temperaturemeasurements to be made and such readings used to calibrate the lowertemperature readings. Thus the gas sensing system herein described canalso be used for calibration of gas measurements at lower temperatures.

It has also been found that by stirring or otherwise agitating theproduct to produce turbulent flow in the pipe 14 in the region of thesensing unit 12 helps to promote the expulsion of SO₂ from the product.This assists the measurement process since for calibrated agitationtechniques more SO₂ is evolved and the higher gas concentrations withinthe unit 12 are more readily measured. However, in using such agitationtechniques it will be appreciated that accurate control measurements ofthe sample will also be required to determine accurately the gasconcentrations within the sample and to calibrate the sensor 50measurements accordingly to take into account the controlled agitationtechniques.

The sensing system 10 can ultimately be used to automate the productionof the preserve. For example, the system 10 may be connected directly toa computer control system which analyses the sensor 50 results tocalculate the gas concentrations within the preserve and, should theconcentration levels rise beyond a predetermined concentration thecontrol system could automatically divert the unacceptable preservefrom, say, a bottling line whilst simultaneously adjusting the heatingof the preserve to increase the rate of "boiling off" of the sulphurdioxide--thereby ensuring that unacceptable preserve is not madeavailable for consumption and rapidly correcting the problem of excesssulphur dioxide concentration to ensure minimum product wastage.

The gas sensor may be regularly calibrated using test samples (of knownconcentrations) of sulphur dioxide injected directly into the chamber 84through one of its ports (not shown) to ensure that the system outputaccurately reflects the gas concentrations within the sensor unitchamber 84.

An additional advantage of the present invention is the ease ofreplacement of the sensor 50 if it becomes damaged by simply dismantlingthe system 10 and replacing the damaged or worn parts.

Although we have discussed sensor systems here which monitor thetemperature and pressure of the product (whether stationary or moving)within the pipe 13, temperature and pressure sensors may also beinserted within the chamber 84 of the sensor unit 12 for measuring thetemperature and pressure within the chamber. Such measurements may beused to further calibrate the sensor system 10 and may also prove usefulin detecting failure of the gas permeable membrane 60 by any suddeninternal pressure or temperature variations. In production models of thepresent invention it has also proved useful to include liquid drainvalves in the chamber 84 design to allow the removal of any condensatewhich may build up over prolonged use.

Further embodiments of the present invention may utilize other methodsof cooling the gas within the chamber 84 (and the body of the system 12and the sensor 50), such as an array of water/fluid cooled pipes withinthe chamber or the use of a water jacket surrounding the chamber towithdraw heat from the gas before the sensor 50 is exposed to it.Alternatively, a Peltier cooling device may be employed within thechamber 84 to withdraw heat from any gas therein. FIG. 4 shows such afurther embodiment of a gas sensing system 10 comprising a more compactarray of sensors than shown in FIG. 5 and employing a water cooledsensor unit. FIG. 4 shows a gas sensing system 210 comprising a mountingblock 213 mounted about a pipe 14. A gas sensing unit 212 of similarconstruction to unit 12 of the first embodiment in FIG. 1 is mounted onthe mounting block and in gaseous communication, via a gas permeablemembrane (not shown) with the pipe 14 contents. However, the heat drainof this unit 212 is connected to a water cooled jacket 220. In additiona pH sensor 222 and a pressure sensor 224 (for measuring the product pHand pressure, respectively, in the pipe 14) are both affixed to themounting block 213. Temperature probes, such as thermocouples orplatinum resistance thermometers, are also inserted into the pipe 213 inthis region for determining the product temperature. The system 10 isthen enclosed within a wire frame cover 230 to protect the pH, gas,pressure and temperature sensors (and the electrical outputs thereof) ofthe system from atmospheric interference which could adversely affectsuch readings. These shielded sensor outputs may then be fed, byshielded cables 232, to the appropriate monitors to measure the gasconcentrations in the sensor 212, and temperature, pressure and pHlevels of the product, and this data interpreted as previously discussedwith reference to the system 10.

It will be appreciated that the present invention is not limited to usein the preserve manufacturing industry or even to the food and drinkindustry and may be used with advantage in a large number of situationswhereby it is necessary to determine the gas (not necessarily sulphurdioxide) content of a flowable medium. A particular application may bein the wine industry again to determine the concentration of sulphurdioxide in a wine; in this use however, it may not be necessary to coolthe gas within the chamber (since the wine need not be hot). It is alsoenvisaged that this invention will have wide application in theproduction, and control, of fruit juices, fruit based beverages andfruit syrups.

It should further be appreciated that the present invention is notrestricted to the determination of sulphur dioxide concentrations and,by use of different gas sensors, the gas sensing system can be appliedfor determining the concentration of an appropriate gas. If required,the sensor system 10 can be adapted to be simply immersed within theflowable medium.

We claim:
 1. A gas sensing system for the detection of gasconcentrations emanating from a flowable medium which comprises amembrane structure permeable to said gas and disposed between saidflowable material and a chamber, said membrane structure beingimpervious to said flowable material; a gas sensor exposed to saidchamber for the detection of concentration of said gas therein, whereinthe gas sensor has an operational temperature limitation less than apredetermined temperature, wherein temperature control means areprovided in the chamber for withdrawing heat from the chamber and saidgas to reduce the gas temperature within the chamber to below thepredetermined temperature.
 2. A system as claimed in claim 1 in whichthe temperature control means comprises a heat sink having a heatconductive member within the chamber connected to heat conducting finsoutside the chamber, in which heat is conducted away from the chamber bythe fins.
 3. A system as claimed in claim 1 in which the sensor has anoperational concentration detection limitation less than a predeterminedconcentration in order to provide increased detection accuracy, andwherein purging means is provided to reduce the concentration of gaswithin the chamber, when necessary, to below the predeterminedconcentration.
 4. A system as claimed in claim 3 in which the chamber issealed and purging means comprises at least two ducts, at least one ductto allow the controlled introduction of a control gas into the chamberand at least one second duct for allowing gas to be removed from thechamber.
 5. A system as claimed in claim 1 which further comprises a pHsensor for detecting the pH level of the flowable medium.
 6. A system asclaimed in claim 5 in which the pH sensor comprises a pH Ion SensitiveField Effect Transistor sensor.
 7. A system as claimed in claim 1 whichfurther comprises pressure detection means for determining the pressureof the flowable medium.
 8. A system as claimed in claim 1 comprising atemperature detection means for determining the temperature of theflowable medium.
 9. A system as claimed in claim 1 in which the flowablemedium flows through a pipe and the membrane structure is situated in awall of said pipe such that a side of the membrane structure, directedinwards towards a central axis of the pipe, is at least partiallyimmersed in the flowable medium.
 10. A system as claimed in claim 9 inwhich the membrane structure is supported by a rigid lattice frame toincrease its strength.
 11. A system as claimed in claim 1 in which thegas sensor is a electrochemical sulphur dioxide detector and themembrane means is permeable to sulphur dioxide.
 12. A system as claimedin claim 1 in which said membrane structure comprises a single layer ora multiplicity of two or more layers.
 13. A method of determining theconcentration of the gas emanating from a flowable medium using a gassensor, comprising the steps of separating and isolating the gas fromthe flowable medium and adjusting the physical characteristics of thesaid isolated gas in order that the gas is compatible with thepredetermined operating ranges of the sensor; whereby the gas isseparated from the flowable medium by placing a membrane structure thatis permeable to the gas and impermeable to the flowable medium betweenthe flowable medium and an isolated chamber to allow the gas to defuseinto the isolated chamber; and where the sensor has an operationaltemperature limitation less than a predetermined temperature wherebyheat is withdrawn from the isolated gas according to temperature controlmeans provided within the isolated chamber to reduce the temperature ofthe gas to below the predetermined temperature.
 14. A method as claimedin claim 13 in which the sensor has an operational concentrationdetection limitation less than a predetermined concentration whereby theconcentration of the isolated gas is prevented from reaching thepredetermined concentration.
 15. A method as claimed in claim 14 inwhich the increase in concentration of the isolated gas is measuredagainst time as the said gas diffuses through a membrane structure in aneffort to reach an equilibrium and the equilibrium concentration of thegas is calculated from the rate of increase of gas concentration beforeequilibrium is reached and the chamber is purged of the gas before thegas concentration reaches the predetermined concentration.